1. Field of the Invention
Our invention is in the field of electroexplosive devices or "initiators," and has particular application in the initiators known as "squibs"--as distinguished from detonators and primers, which differ mostly in having greater explosive energy.
The present invention is directed to rendering a squib or other initiator substantially insensitive to stray radio-frequency electromagnetic fields in the ambient surrounds, and also to electrostatic charge accumulated by ambient phenomena, so that the squib reliably responds only to applied signals of the design voltage and wattage.
This invention has principal application in squibs having two connection terminals and an electrically isolated casing, but as explained below can be used to advantage under certain circumstances in a "coaxial" squib--the type in which the casing forms one electrical terminal.
2. Prior Art
Many theories have been advanced for the observed tendency of initiators, on occasion, to ignite without deliberate signal application, and in response to those theories many squib designs have resulted purportedly eliminating such spurious ignition.
Indeed inadvertent ignition seems to have been eliminated by a number of squib designs, but only by incorporation of features which are unacceptable in one or another application. In some instances accidental ignition is eliminated in a particular application or in particular types of circumstances but not in others.
Thus a considerable array of specialized squibs has been developed, in which design complexity, production cost, size, electrical and explosive characteristics, materials of construction, and reliability of firing in response to intentionally applied signals--as well as reliability of nonfiring in the absence of such intentionally applied signals--are mutually traded off to best advantage for particular applications.
The instant invention is a response to a specialized application wherein many of the most-stringent constraints of prior applications are present in combination, and so is in a sense a highly specialized squib. However, it is sufficiently easy and economical of manufacture and use to be an acceptable substitute for many initiators whose constraints are not so severe. In this sense, therefore, the present invention may be considered to constitute an advantageous "general-purpose," though not "universal," squib.
The simplest prior-art squib design, and ironically one of the most reliable designs as to RF-insensitivity, is the single-pin "coaxial" or "coax" type mentioned earlier. Such a squib consists of a generally cylindrical case having a generally coaxially mounted terminal, and a heater wire, called a "bridge" or "bridgewire," (or other heater structure) electrically connected between the terminal and case and in thermal contact with the explosive charge, be it pyrotechnic or higher-power explosive material. When used with pyrotechnic material (as opposed to primary-explosive powder) such a design is relatively insensitive to ignition by RF-induced sparks, because RF-induced voltages between pin and case are leaked off in the form of low-amperage current through the bridgewire. Much higher current levels are required to achieve the high temperatures at which pyrotechnic (metal and oxidant) formulations ignite.
There are, however, two limiting characteristics of the coax squib. First, it should not be a foregone conclusion that RF voltages always leak off across the bridge structure to the case, as this is a matter which involves the impedance of the particular bridge structure over an enormous range of RF frequencies as well as nearly unlimited variability in the orientation, polarization and power level of stray RF fields. Under just certain operating conditions it may be possible for an RF field to excite a coax-squib bridge at a frequency for which the bridge--in its squib-casing environment--is not a low impedance at all, but a very high impedance. Under these circumstances the fields may well result in an RF discharge which bypasses the bridge, igniting the explosive charge. It will be recalled that a small straight wire acts as an inductor, not a short, to radio-frequency power. It is entirely conceivable that such a wire in a particular orientation in a particular coax casing could form a tuned resonant circuit to RF power of a particular frequency, standing off the voltage and producing ignition as described above. Such phenomena may become even more likely where elaborate special geometries are employed to solve special problems in coax squibs.
For example, U.S. Pat. No. 3,867,885, assigned to Dynamit Nobel Aktiengesellschaft represents a very elaborate coax squib in which the bridge structure is metal plated or coated on an insulating disc, rather than being an initially integral wire. Thus the bridge has an exceedingly thin but relatively wide cross-section. It is disposed from a central pin or plated-through cavity radially to an annular contact ring created by the same process. At the other end of the central pin or central plated-through cavity is a circular metal disc likewise plated or coated on the bottom of the insulating disc. The insulating disc carrying the metal plated disc is placed in contact with a massive cylindrical "pole piece" which nearly surrounds the disc and plated bridge structure. A metal washer above the insulating disc makes contact with the annular contact ring previously mentioned. It would be virtually impossible to ascertain the response of this structure to all RF frequencies and field orientations which might be encountered under actual operating conditions, especially considering modifications of the thin plated-on structure which might preliminarily be caused by high-power RF fields. Thus the device is not in fact guaranteed RF-proof.
It may be noted in passing that the device of U.S. Pat. No. 3,867,885 incorporates a cup-shaped device bearing superficial similarity to a structure in a preferred embodiment of the instant invention. However, the cup-shaped device in the referenced patent is directly attached electrically to the squib casing, with no spark gap, and cannot function in the way or for the purpose described hereunder for the similar-appearing structure of the instant invention.
The second limitation of coax squibs is their susceptibility to inadvertent ignition due to an entirely different kind of accident:
Normally the case of a coax squib is placed at chassis ground of the weapon, spaceflight module, vehicle or other apparatus in which it is used, by gripping in a simple grounded receptacle. The firing signal then is applied with respect to chassis ground. The problem is that the squib can be fired by accidental touching of numerous other "hot" wires in the apparatus to the squib terminal or its signal wire, at any point in the apparatus. This can occur, for example, by a hand tool falling across two points in the circuit--or, perhaps more seriously, by a portion of the apparatus structure coming loose or sagging or being bent by unanticipated external impacts, so as to short a hot wire to the squib signal line. Just such sensitivities as this make coax squibs unacceptable in, for example, automotive air-bag inflators--where many years of use in automotive environments may readily cause just such accidents to occur.
Applications such as equipment deployment in high-reliability spaceflight vehicles, or the automotive air-bag inflator just mentioned, have given rise to the two-pin squib with floating case, in which the firing signal may be applied through an electrically isolated, floating circuit encompassing the two squib terminals. Even if the circuit is not completely "floated," a relative insensitivity to contact with the normal "hot" wires of nearby circuits can be obtained by judicious selection of voltage levels for the two signal wires to the squib.
Two-terminal, floating-case squibs may be typified by that in FIG. 1A of U.S. Pat. No. 3,783,788, assigned to Nippon Oils and Fats Company Ltd. of Tokyo. In two-terminal squibs the bridgewire, or some other igniting device, is connected between the two terminals rather than between a terminal and the case. Such devices are suggested as well by the prior-art schematic presented as FIG. 1 hereof: the case 11 has mounted within it two terminals 22 and 23, and bridgewire 21. The comments above relating to leakage across the bridgewire (from terminal to case in a coax squib) are applicable here as well (from terminal to terminal) in a two-terminal squib. That is to say, RF sparking between the terminals or pins 22 and 23 is relatively unlikely because voltage tends to be leaked off across the bridgewire--but is still possible inasmuch as the bridge structure in its squib-case environment may form a high impedance for certain RF fields, leading to a spark paralleling the bridge. In the two-terminal squib, however, there is an entirely new problem of RF arcing between either terminal 22 or 23 and the case 11, much more severe than in the coax squib, because normally there is no current leakage path to the case: by definition, the case is floating. If neither electrostatic nor radio-frequency energy is dissipated by low-current leakage, sufficient voltage of either sort can develop to cause a spark discharge within the case, as at 31 (FIG. 1), or at an externally provided safety spark gap 32, formed between one terminal 22 and an inward-extending case portion 12. To provide spatial separation within the squib case reliably capable of preventing high-voltage arcs, squibs several inches across would be required, straining both material costs and space requirements.
(While the squib case is commonly described as "floating," this terminology is intended only to mean that the case is floating with respect to the firing circuit. As to RF fields the case readily forms part of an induction loop, and as to electrostatic voltages the case in its operating environment is likely to be securely grounded or effectively at ground, or chassis ground, potential.)
U.S. Pat. No. 3,783,788 offers one ostensible solution to the electrostatic part of this problem--an electrically "leaky" insulator forming the seal between the case interior and ambient. Electrostatic charge accumulating between the pins, or between either pin and the case, dissipates by low-level current flow through the insulator. This may work quite well for reasonably gradual electrostatic accumulation, but it is not likely to offer sufficiently low impedance to prevent RF-induced sparks from terminal to case. Here again as it happens there is a superficial similarity between the internal structure "c" of the referenced patent and a part of the preferred embodiment of the instant invention. However, the structure "c" of the referenced patent is in firm electrical contact with the outer case, and is connected to neither of the two terminals; its function relates to a staging of the ignition of the various explosive charges, rather than to any spark-relief feature.
U.S. Pat. No. 4,061,088 offers another solution to the electrostatic-spark problem: ganged zener diodes between the terminals and between each terminal and the case. These diodes offer operation superior to that in the previous patent discussed, in that deliberately applied ignition signals, being below the zener threshold, are not leaked off and degraded; whereas high-voltage electrostatic charges exceed the zener threshold and are selectively dissipated. This patent represents the most recent in a sequence of progressively more specialized patents to squibs with voltage-discriminating dissipation paths, the first of which issued in 1937 as number U.S. Pat. No. 2,086,548. There are two limitations to the zener-diode approach: first, the behavior of zener diodes in response to RF induced voltages, in the confines of a squib case, is a matter for considerable conjecture or investigation; and, second, the cost of semiconductor devices of this sort may be excessive for many applications. As to the zener RF characteristic, it will be clear that if the zener diode is not fast enough to turn on and conduct sufficient electricity during a given RF half-cycle, that alone would be sufficient to negate the device's beneficial effect. More serious is the question whether the RF field "sees" the zener diode as a conductor at all, and if so whether as a resistive or reactive conductor--and if reactive, to what extent the zener might stand off RF voltage (as suggested in the case of the bridgewire, earlier), permitting a parallel spark. In short, the zener-diode-fitted squib may be totally insensitive to electrostatic interference but still quite sensitive to RF interference.
Another possibility, not described in the referenced patent but explored by the present inventors, is use of a "lossy" RF filter installed in essentially the location of the zener diodes in the previously discussed patent. It has been conclusively demonstrated that such "lossy" RF filters can provide a completely reliable leakage path for all RF induced voltages. However, as with the zener diodes, these filters are expensive; in fact, for one particular design we found that even in extremely large production quantities (e.g., millions, for automotive applications) the filters alone would cost in the neighborhood of $1.50 (1978 value). Thus the filters alone would cost roughly as much as the rest of the squib, doubling the squib cost. Such cost is generally considered unacceptable, outside of the military, spaceflight, or luxury-item fields.
Neither zener-diode and lossy-RF-filter design provides any spark-gap diversion path; both rely instead on the electronic characteristics of the respective components.
U.S. Pat. No. 3,274,937 discloses a spark gap in a two-terminal squib. However, this gap is provided for a different purpose and in a different location and fashion than the gap of the present invention, to be described hereunder. The gap of the referenced patent is in series with the ignition circuit (in this case, a bridgewire), and is intended "to make the detector immune to applied voltages below a certain critical voltage" (emphasis supplied), whereas as will be clear from the description hereunder the gap of the present invention is not in series with the ignition circuit but rather is between that circuit and the squib case, and is designed to make the squib insensitive to voltages above a threshold voltage. As a matter of fact, the referenced patent discloses no particular apparent protection against sparking between the pins and the case, especially as to the nongapped pin.
U.S. Pat. No. 3,257,946 discloses a two-pin, floating-case squib intended to display RF insensitivity, voltage and energy discrimination, and amenability to testing both without and with explosive charge in place, without firing. This device comprises a two-stage charge, with the priming charge separated from a series spark gap by a thin metal membrane. A spark in the gap, if of sufficient intensity, ruptures the membrane, thereby exposing the priming charge to the spark. The spark ignites the priming charge, which in turn ignites the main charge. Though this configuration is described as RF-insensitive, that purported characteristic is said to result from the series spark gap in one terminal pin, and the resulting voltage threshold for firing, coupled with the membrane barrier and its resulting energy threshold for firing. Thus by interference the insensitivity is to interpin RF induction, not pin-to-case induction. Consideration of the geometry suggests that the device may in fact not be at all protected against pin-to-case sparking, particularly via the ungapped pin or the membrane itself.
U.S. Pat. No. 3,971,320 shows a hybrid squib which seems to have coax and dual-pin advantages. It is a coax unit to whose metal case a second pin is electrically connected; the squib is in a plastic outer casing, with a sealed mouth penetrated by terminals. This does not give the coax's relative RF-insensitivity, with the floating case's relative immunity to accidental shorting. Unless the "mounting" which holds the squib, and other items within an inch, are dry, clean nonmetal, the squib is susceptible to arcing--from the inner metal case through plastic to whatever conductor is in striking range. The metal case, connected to one pin, can act just as do the pins in a two-pin floating-case squib, in arcing to the nearest equivalent of the floating case. Of course the reference squib is also as susceptible as any coax squib to interpin RF sparking. The most serious drawback of the reference squib is that the interior and exterior surfaces of the insulating casing 10 cannot be reliably sealed to the inner cup 12 (or the mouth seal) or the aforementioned mounting, respectively. (In many applications the squib itself forms part of a sealed system into which its ignition products are discharged.) This permits two longitudinal leakage paths to arise along the respective annular interfaces, each path passing contaminants inward and hot pyrotechnic gases outward. High-strength seals (welds, solder, compression glass, or threads) function adeuquately only with metal, noninsulating outer cases.
The foregoing discussion generally exhausts the closest related prior art of which we are aware. However, it may be useful also to consider some hypothetical geometries not known or suggested by the prior-art references, but which may be regarded as constructs produced by combining certain features of various references. In particular, single-pin coax squibs could be produced having the spark-gap and membrane of U.S. Pat. No. 3,257,946, but with the return simply connected to the case instead of a second terminal. That is, the metal membrane could be connected to the case. Such a device would of course be susceptible to accidental shorts, as is any coax squib, but it would be susceptible to membrane-to-case sparks, since the membrane would be connected to the case. The remaining question is whether it would be susceptible to sparks between the single terminal and the case. Presumably such sparks would form in the series spark gap provided for ignition sparks. If a parallel, diversion gap were defined in parallel between that terminal and the case, and if the diversion gap operated at a lower voltage than the series gap, then the device would be unfirable. If the diversion gap operated at a higher voltage than the series gap, then the diversion gap would never fire and might as well be omitted. In short, the device hypothesized is not protectable against RF overvoltage across the series spark gap. Concededly such protection might not be required, since RF-induced sparks in that gap would typically be extremely low-current sparks, incapable of rapidly piercing the membrane to gain access to the explosive charge. However, it is possible that if the squib happened to be exposed to RF fields on an essentially continuous basis, as could occur in an automotive environment (to take a circumstance in pertinent point), the low-current sparks could cumulatively degrade the membrane over a period of months or years to the point where the membrane failed. To avoid this result might require making the membrane so thick that it would not rupture rapidly enough, under application of an intentional firing signal, to provide necessary protective time response in a vehicle safety air-bag inflator or the like. In summary, a coax squib with series spark-gap and energy-discriminating membrane would have the usual susceptibility to shorting accidents of all coax squibs, and to avoid cumulative deterioration (and accidental firing) due to RF exposure might well have to have excessively slow response.
Another possibility would be to combine the "leaky" insulator of U.S. Pat. No. 3,783,788 with a coaxial geometry. The referenced patent, as may be recalled, discloses a bridgewire type of ignition means; consequently the hybrid here suggested would have the same possibility, though perhaps remote, of RF sparking in parallel with the bridgewire as any other coax squib with a bridgewire--as previously discussed. The "leaky" insulation would not be any more effective in leaking RF voltage in the coax configuration than in the two-pin configuration. And the device would of course be susceptible to accidental shorting of the single pin to a "hot" circuit element, as also previously described.
One other prior-art feature is worthy of mention, namely the provision of a spark gap between pins and case on the exterior side of a squib seal--that is to say, exposed to ambient. Such a gap is suggested at 32 in FIG. 1. Because such spark gap is so exposed, it is subject to deterioration by mechanical damage, by accumulation of dirt, or by corrosion or oxidation of the surfaces involved. While it may be unlikely that such deterioration could prevent proper bypassing of a high-voltage RF spark, it could result in similar bypassing of a deliberately applied firing signal. Consequently, exposed pin-to-case protective spark gaps are not highly regarded.
When all the constraints discussed in the preceding pages are considered in combination--constraints of response time, cost, size, reliable firing on command, and above all reliable nonfiring in the presence of (1) stray RF fields, (2) electrostatic phenomena and (3) mechanical mishaps--it becomes clear that no prior-art squib adequately satisfies the combined constraints. Just such a combination of constraints characterizes the requirements of the automotive safety devices mentioned earlier.